Oxidative stress plays an important role in the progression of neurodegenerative and age-related diseases, causing damage to proteins, DNA, and lipids. Low molecular weight, hydrophobic antioxidant compounds are useful in treating conditions of peripheral tissues, such as acute respiratory distress syndrome, amyotrophic lateral sclerosis, atherosclerotic cardiovascular disease, multiple organ dysfunctions and central nervous system neurodegenerative disorders, e.g., Parkinson's disease, Alzheimer's disease and Creutzfeldt-Jakob's disease. Oxidative stress has been causally linked to the pathogenesis of Parkinson's disease, Alzheimer's disease and Creutzfeldt-Jakob's disease, as well as other types of disorders. (U.S. Pat. No. 6,420,429 to D. Atlas et al.).
A deficiency of cellular antioxidants may lead to excess free radicals, which cause macromolecular breakdown, lipid peroxidation, buildup of toxins and ultimately cell death. Because of the importance of antioxidant compounds in preventing this cellular oxidation, natural antioxidants, such as glutathione (GSH) (γ-glutamyl cysteinyl glycine) are continuously supplied to the tissues. GSH is synthesized by most cells and is one of the primary cellular antioxidants responsible for maintaining the proper oxidation state within the body. When oxidized, GSH forms a dimer, GSSG, which may be recycled in organs producing glutathione reductase. In human adults, reduced GSH is produced from GSSG, primarily in the liver, and to a smaller extent, by skeletal muscle and red and white blood cells, and is distributed through the blood stream to other tissues in the body.
However, under certain conditions, the normal, physiologic supplies of GSH are insufficient, its distribution is inadequate or local oxidative demands are too high to prevent cellular oxidation. Under other conditions, the production of and demand for cell antioxidants, such as GSH, are mismatched, thus leading to insufficient levels of these molecules in the body. In other cases, certain tissues or biological processes consume the antioxidants so that their intracellular levels are suppressed. In either case, increased serum levels of antioxidant, e.g., glutathione, leads to increased amounts of the antioxidant that can be directed into cells. In facilitated transport systems for cellular uptake, the concentration gradient that drives uptake is increased.
Glutathione N-acetylcysteine amide (NAC amide), the amide form of N-acetylcysteine (NAC), is a low molecular weight thiol antioxidant and a Cu2+ chelator. NAC amide provides protective effects against cell damage. NAC amide was shown to inhibit tert.-butylhydroxyperoxide (BuOOH)-induced intracellular oxidation in red blood cells (RBCs) and to retard BuOOH-induced thiol depletion and hemoglobin oxidation in the RBCs. This restoration of thiol-depleted RBCs by externally applied NAC amide was significantly greater than that found using NAC. Unlike NAC, NAC amide protected hemoglobin from oxidation. (L. Grinberg et al., Free Radic Biol Med., 2005 Jan. 1, 38(1):136-45). In a cell-free system, NAC amide was shown to react with oxidized glutathione (GSSG) to generate reduced glutathione (GSH). NAC amide readily permeates cell membranes, replenishes intracellular GSH, and, by incorporating into the cell's redox machinery, protects the cell from oxidation. Because of its neutral carboxyl group, NAC amide possesses enhanced properties of lipophilicity and cell permeability. (See, e.g., U.S. Pat. No. 5,874,468 to D. Atlas et al.). NAC amide is also superior to NAC and GSH in crossing the cell membrane, as well as the blood-brain barrier.
NAC amide may function directly or indirectly in many important biological phenomena, including the synthesis of proteins and DNA, transport, enzyme activity, metabolism, and protection of cells from free-radical mediated damage. NAC amide is a potent cellular antioxidant responsible for maintaining the proper oxidation state within the body. NAC amide can recycle oxidized biomolecules back to their active reduced forms and may be as effective, if not more effective, than GSH as an antioxidant.
Glutamate, an excitatory amino acid, is one of the major neurotransmitters in the central nervous system (CNS). Elevated levels of extracellular glutamate have been shown to be responsible for acute neuronal damage as well as many CNS disorders, including hyperglycemia, ischemia, hypoxia (Choi, D. W., Neuron, 1(8):623-34, 1988), and chronic disorders such as Huntington's, Alzheimer's, and Parkinson's diseases (Meldrum B. and Garthwaite J., Trends Pharmacol Sci., 11(9):379-87, 1990; and Coyle J. T. and Puttfarcken P., Science, 262(5134):689-95, 1993). Two mechanisms have been proposed for glutamate toxicity. The first mechanism explains the excitotoxicity of glutamate as being mediated through three types of excitatory amino acid receptors (Monaghan D. T. et al., Annu Rev Pharmacol Toxicol., 29:365-402, 1989). In addition to receptor-mediated glutamate excitotoxicity, it has also been proposed that elevated levels of extracellular glutamate inhibits cystine uptake, which leads to a marked decrease in cellular GSH levels, resulting in the induction of oxidative stress (Murphy T. H. et al., Neuron, 2(6):1547-58, 1989).
Cysteine is a critical component for intracellular GSH synthesis. Because of redox instability, almost all of the extracellular cysteine is present primarily in its oxidized state, cystine, which is taken up by cells via a cystine/glutamate transporter, the X c-system. Studies indicate that glutamate and cystine share the same transporter; therefore, elevated levels of extracellular glutamate competitively inhibit cystine transport, which leads to depletion of intracellular GSH. (Bannai S. and Kitamura E., J Biol. Chem. 255(6):2372-6, 1980; and Bannai S., Biochem Biophys Acta., 779(3):289-306, 1984). Depletion of reduced glutathione results in decreased antioxidant capacity of the cell, accumulation of ROS (reactive oxygen species), and ultimately apoptotic cell death. Several studies have demonstrated the induction of oxidative stress by glutamate in various cell lines including immature cortical neurons (Murphy T. H. et al., FASEB J., 4(6):1624-33, 1990; and Sagara J. et al., J Neurochem., 61(5):1667-71, 1993), oligodendroglia (Oka A. et al., J Neurosci., 13(4):1441-53, 1993), cultured rat astrocytes (Cho Y. and Bannai S., J Neurochem., 55(6):2091-7, 1990), neuroblastoma cells (Murphy T. H. et al., Neuron., 2(6):1547-58, 1989), and PC12 cells (Froissard P. and Duval D., Neurochem Int., 24(5):485-93, 1994).
Certain antioxidants such as NAC, lipoic acid (LA), (Han D. et al., Am J Physiol., 273:1771-8, 1997), tocopherol (Pereira C. M. and Oliveira C. R., Free Radic Biol Med., 23(4):637-47, 1997), and probucol (Naito M. et al., Neurosci Lett., 186(2-3):211-3, 1995) can protect against glutamate cytotoxicity, mostly by replenishing GSH. However, in certain neurological diseases, such as cerebral ischemia and Parkinson's disease, enhancement of tissue GSH in brain regions cannot be attained, because these antioxidant agents have been obstructed by the blood-brain barrier (Panigrahi M. et al., Brain Res., 717(1-2):184-8, 1996; and Gotz M. E. et al., J Neural Transm Suppl., 29:241-9, 1990).
In addition to neurodegenerative diseases, such as those which affect the brain and/or peripheral nervous tissues, other diseases, such as asthma, respiratory-related diseases and conditions, e.g., acute respiratory distress syndrome (ARDS), amyotrophic lateral sclerosis (ALS or Lou Gerhig's disease), atherosclerotic cardiovascular disease and multiple organ dysfunction, are related to the overproduction of oxidants or reactive oxygen species by cells of the immune system.
A number of other disease states have been specifically associated with reductions in the levels of antioxidants such as GSH. Depressed antioxidant levels, either locally in particular organs or systemically, have been associated with a number of clinically defined diseases and disease states, including HIV/AIDS, diabetes and macular degeneration, all of which progress because of excessive free radical reactions and insufficient antioxidants. Other chronic conditions may also be associated with antioxidant deficiency, oxidative stress, and free radical formation, including heart failure and associated conditions and pathologies, coronary arterial restenosis following angioplasty, diabetes mellitus and macular degeneration.
Clinical and pre-clinical studies have demonstrated the linkage between a range of free radical disorders and insufficient antioxidant levels. It has been reported that diabetic complications are the result of hyperglycemic episodes that promote glycation of cellular enzymes and thereby inactivate the synthetic pathways of antioxidant compounds. The result is antioxidant deficiency in diabetics, which may be associated with the prevalence of cataracts, hypertension, occlusive atherosclerosis, and susceptibility to infections in these patients.
High levels of antioxidants, such as GSH, have been demonstrated to be necessary for proper functioning of platelets, vascular endothelial cells, macrophages, cytotoxic T-lymphocytes, and other immune system components. Recently it has been discovered that patients infected with the human immunodeficiency virus, HIV, exhibit low GSH levels in plasma, other body fluids, and in certain cell types, such as macrophages. These low GSH levels do not appear to be due to defects in GSH synthesis. Antioxidant deficiency has been implicated in the impaired survival of patients with HIV. (1997, PNAS USA, Vol. 94, pp. 1967-1972). Raising antioxidant levels in cells is widely recognized as being important in HIV/AIDS and other disorders, because the low cellular antioxidant levels in these disease types permit more and more free radical reactions to fuel and exacerbate the disorders.
HIV is known to start pathologic free radical reactions, which lead to the destruction of antioxidant molecules, as well as their exhaustion and the destruction of cellular organelles and macromolecules. In mammalian cells, oxidative stresses, e.g. low intracellular levels of reduced antioxidants and relatively high levels of free radicals, activate certain cytokines, including NF-κB and TNF-α, which, in turn, activate cellular transcription of the DNA to mRNA, resulting in translation of the mRNA to a polypeptide sequence. In a virus-infected cell, the viral genome is transcribed, resulting in viral RNA production, generally necessary for viral replication of RNA viruses and retroviruses. These processes require a relatively oxidized state of the cell, a condition which results from stress, low antioxidant levels, or the production of reduced cellular products. The mechanism which activates cellular transcription is evolutionarily highly conserved, and therefore it is unlikely that a set of mutations would escape this process, or that an organism in which mutated enzyme and receptor gene products in this pathway would be well adapted for survival. Thus, by maintaining a relatively reduced state of the cell (redox potential), viral transcription, a necessary step in late stage viral replication, is impeded.
The amplification effect of oxidative intracellular conditions on viral replication is compounded by the actions of various viruses and viral products, which degrade antioxidants, such as GSH. For example, gp120, an HIV surface glycoprotein having a large number of disulfide bonds, is normally present on the surface of infected cells. gp120 oxidizes GSH, resulting in reduced intracellular GSH levels. On the other hand, GSH reduces the disulfide bonds of gp120, thus reducing or eliminating its biological activity that is necessary for viral infectivity. Antioxidants such as GSH therefore interfere with the production of such oxidized proteins and degrade them once formed. In a cell that is actively replicating viral gene products, a cascade of events may occur which can allow the cell to pass from a relatively quiescent stage with low viral activity to an active stage with massive viral replication and cell death. This is accompanied by a change in redox potential. By maintaining adequate levels of antioxidant, this cascade may be impeded.
HIV is transmitted through two predominant routes, namely, contaminated blood and/or sexual intercourse. In pediatric cases, approximately one half of the newborn individuals are infected in utero and one half are infected at delivery. This circumstance permits a study of prevention of transmission since the time of spread is known. Initially, there is an intense viral infection simulating a severe case of the flu, with massive replication of the virus. Within weeks, this acute phase passes spontaneously as the body mounts a largely successful immune defense. Thereafter, the individual has no outward manifestations of the infection. However, the virus continues to replicate within immune system cells and tissues, e.g., lymph nodes, lymphoid nodules, macrophages and certain multidendritic cells that are found in various body cavities.
Such stealthy and widespread infection is not just a viral problem. The virus, in addition to replicating, causes excessive production of various free radicals and various cytokines in toxic or elevated levels. The cytokines are normally occurring biochemical substances that signal numerous reactions and that typically exist in minuscule concentrations. Eventually, after an average of 7-10 years of seemingly quiescent HIV infection, the corrosive free radicals and the toxic levels of cytokines begin to cause outward symptoms in infected individuals and failures in the immune system begin. Substances like 15-HPETE are immunosuppressive and TNF-α causes muscle wasting, among other toxic factors. The numbers of viral particles increase and the patient develops the Acquired Immune Deficiency Syndrome, AIDS, which may last 2 to 4 years before the individual's demise. AIDS, therefore, is not merely a virus infection, although the viral infection is believed to be an integral part of the etiology of the disease.
Further, HIV has a powerful ability to mutate. It is this capability that makes it difficult to create a vaccine or to develop long-term, antiviral pharmaceutical treatments. As more people fail to be successfully treated by the present complex regimens, the number of resistant viral strains is increasing. Resistant strains of HIV are a particularly dangerous population of the virus and pose a considerable health threat. These resistant HIV mutants also add to the difficulties in developing vaccines that will be able to inhibit the activity of highly virulent viral types. Further, the continuing production of free radicals and cytokines that may become largely independent of the virus perpetuate the dysfunctions of the immune system, the gastrointestinal tract, the nervous system, and many other organs in patients with AIDS. The published scientific literature indicates that many of these diverse organ system dysfunctions are due to systemic deficiencies of antioxidant compounds that are engendered by the virus and its free radicals. For example, GSH is consumed in HIV infections because it is the principal, bulwark antioxidant versus free radicals. An additional cause of erosion of GSH levels is the presence of numerous disulfide bonds in HIV proteins, such as the gp120 cell surface protein. Disulfide bonds react with GSH and oxidize it. Thus, there is a need for other antioxidants to be used to replace antioxidants such as GSH whose normal function is adversely affected by HIV infection.
The current HIV/AIDS pharmaceuticals take good advantage of the concept of pharmaceutical synergism, wherein two different targets in one process are affected simultaneously. The effect is more than additive. The drugs now in use were selected to inhibit two very different points in the long path of viral replication. The pathway of viral replication as understood by skilled practitioners in the art is described in U.S. Pat. No. 6,420,429. New anti-HIV/AIDS therapies include additional drugs in the classes of Reverse Transcriptase inhibitors and protease inhibitors. Also, drugs are in development to block the integrase enzyme of the virus, which integrates the HIV DNA into the infected cell's DNA, analogous to splicing a small length of wire into a longer wire. Vaccine development also continues, although prospects seem poor because HIV appears to be a moving target and seems to change rapidly. Vaccine development is also impaired by the immune cell affinity of the virus.
Individuals infected with HIV have lowered levels of serum acid-soluble thiols and antioxidants such as GSH in plasma, peripheral blood monocytes and lung epithelial lining fluid. In addition, it has been shown that CD4+ and CD8+ T cells with high intracellular GSH levels are selectively lost as HIV infection progresses. This deficiency may potentiate HIV replication and accelerate disease progression, especially in individuals with increased concentrations of inflammatory cytokines, because such cytokines stimulate HIV replication more efficiently in cells in which antioxidant compounds are depleted. In addition, the depletion of antioxidants, such as GSH, is also associated with a process known as apoptosis, or programmed cell death. Thus, intercellular processes which artificially deplete GSH may lead to cell death, even if the process itself is not lethal.
Diabetes mellitus (“diabetes”) is found in two forms: childhood or autoimmune (Type I, IDDM) and late-onset or non-insulin dependent (Type II, NIDDM). Type I constitutes about 30% of the cases of diabetes. The rest of the cases are represented by Type II. In general, the onset of diabetes is sudden for Type I and insidious or chronic for Type II. Symptoms include excessive urination, hunger and thirst, with a slow and steady loss of weight associated with Type I. Obesity is often associated with Type II and has been thought to be a causal factor in susceptible individuals. Blood sugar is often high and there is frequent spilling of sugar in the urine. If the condition goes untreated, the victim may develop ketoacidosis with a foul-smelling breath similar to some who has been drinking alcohol. The immediate medical complications of untreated diabetes can include nervous system symptoms, and even diabetic coma.
Because of the continuous and pernicious occurrence of hyperglucosemia (very high blood sugar levels), a non-enzymatic chemical reaction, called glycation, frequently occurs inside cells and causes a chronic inactivation of essential enzymes. One of the most critical enzymes, γ-glutamyl-cysteine synthetase, is glycated and readily inactivated. This enzyme is involved in a critical step in the biosynthesis of glutathione in the liver. The net result of this particular glycation is a deficiency in the production of GSH in diabetics.
GSH is in high demand throughout the body for multiple, essential functions, for example, within all mitochondria, to produce chemical energy called ATP. With a deficiency or absence of GSH, brain cells, heart cells, nerve cells, blood cells and many other cell types are not able to function properly and can be destroyed through apoptosis associated with oxidative stress and free radical formation. GSH is the major antioxidant in the human body and the only one that can be synthesized de novo. It is also the most common small molecular weight thiol in both plants and animals. Without GSH the immune system cannot function, and the central and peripheral nervous systems become aberrant and then cease to function. Because of the dependence on GSH as the carrier of nitric oxide, a vasodilator responsible for control of vascular tone, the cardiovascular system does not function well and eventually fails. Since all epithelial cells seem to require GSH, without GSH, intestinal lining cells also do not function properly and valuable micronutrients are lost, nutrition is compromised, and microbes are given portals of entry to cause infections.
In diabetes, the use of GSH precursors cannot help to control GSH deficiency due to the destruction of the rate-limiting enzyme by glycation. As GSH deficiency becomes more profound, the well-known sequelae of diabetes progress in severity. The complications that develop in diabetics are essentially due to runaway free radical damage since the available GSH supplies in diabetics are insufficient. For example, a diabetic individual becomes more susceptible to infections because the immune system approaches collapse when GSH levels fall, analogous to the situation in HIV/AIDS. In addition, peripheral vasculature becomes comprised and blood supply to the extremities is severely diminished because GSH is not available in sufficient amounts to stabilize nitric oxide to effectively exert its vascular dilation (relaxation) property. Gangrene is a common sequel and successive amputations often result in later years. Peripheral neuropathies, the loss of sensation commonly of the feet and lower extremities develop and are often followed by aberrant sensations like uncontrollable burning or itching. Retinopathy and nephropathy are later events that are actually due to microangiopathy, i.e., excessive budding and growth of new blood vessels and capillaries, which often will bleed due to weakness of the new vessel walls. This bleeding causes damage to the retina and kidneys with resulting blindness and renal shutdown, which requires dialysis treatment. Further, cataracts occur with increasing frequency as the GSH deficiency deepens. Large and medium sized arteries become sites of accelerated severe atherosclerosis, with myocardial infarcts at early ages, and of a more severe degree. If coronary angioplasty is used to treat the severe atherosclerosis, diabetics are much more likely to have re-narrowing of cardiac vessels, termed restenosis.
Macular degeneration as a cause of blindness is a looming problem as the population ages. Age-related macular degeneration (ARMD) is characterized by either a slow (dry form) or rapid (wet form) onset of destruction and irrevocable loss of rods and cones in the macula of the eye. The macula is the approximate center of the retina wherein the lens of the eye focuses its most intense light. The visual cells, known as the rods and cones, are an outgrowth and active part of the central nervous system. They are responsible and essential for the fine visual discrimination required to see clear details such as faces and facial expression, reading, driving, operation of machinery and electrical equipment and general recognition of surroundings. Ultimately, the destruction of the rods and cones leads to functional, legal blindness. Since there is no overt pain associated with the condition, the first warnings of onset are usually noticeable loss of visual acuity. This may already signal late stage events. It is now thought that one of the very first events in this pathologic process is the formation of a material called “drusen”, which first appears as either patches or diffuse drops of yellow material deposited upon the surface of the retina in the macula lutea or yellow spot. This is the area of the retina where sunlight is focused by the lens and which contains the highest density of rods for acuity. Although cones, which detect color, are lost as well in this disease, it is believed to be loss of rods, which causes the blindness. Drusen has been chemically analyzed and found to be composed of a mixture of lipids that are peroxidized by free radical reactions.
It is believed that the loss of retinal pigmented epithelial (RPE) cells occurs first in ARMD. Once an area of the retinal macula is devoid of RPE cells, loss of rods, and eventually some cones, occurs. Finally, budding of capillaries begins and typical microangiopathy associated with late stage ARMD occurs. It is also known that RPE cells require large quantities of GSH for their proper functioning. When GSH levels drop severely in cultures of RPE cells, the RPE cells begin to die. When cultures of these cells are supplemented with GSH in the medium, they thrive. There is increasing evidence that progression of the disease is paced by a more profound deficiency in GSH within the retina and probably within these cells, as indicated by cell culture studies.
It is generally believed that “near” ultraviolet (UVB) and visual light of high intensity primarily from sunlight is a strong contributing factor of ARMD. People with light-colored irises constitute a high risk population for macular degeneration, as do those with jobs that keep them outdoors and those in equatorial areas where sunlight is most intense. Additional free radical insults, e.g., smoking, adds to the risk of developing ARMD. Several approaches have been unsuccessfully tested to combat ARMD, including chemotherapy. Currently, there is no effective therapy to treat ARMD. Laser therapy has been developed which has been used widely to slow the damage produced in the slow onset form of the disease by cauterizing neovascular growth. However the eventual outcome of the disease, once it has started to progress, is certain.
The importance of thiols and especially of GSH to lymphocyte function has been known for many years. Adequate concentrations of GSH are required for mixed lymphocyte reactions, T-cell proliferation, T- and B-cell differentiation, cytotoxic T-cell activity, and natural killer cell activity. Adequate GSH levels have been shown to be necessary for microtubule polymerization in neutrophils. Intraperitoneally administered GSH augments the activation of cytotoxic T-lymphocytes in mice, and dietary GSH was found to improve the splenic status of GSH in aging mice, and to enhance T-cell mediated immune responses. The presence of macrophages can cause a substantial increase of the intracellular GSH levels of activated lymphocytes in their vicinity. Macrophages consume cystine via a strong membrane transport system, and generate large amounts of cysteine, which they release into the extracellular space. It has been demonstrated that macrophage GSH levels (and therefore cysteine equivalents) can be augmented by exogenous GSH. T-cells cannot produce their own cysteine, and it is required by T-cells as the rate-limiting precursor of GSH synthesis. The intracellular GSH level and the DNA synthesis activity in mitogenically-stimulated lymphocytes are strongly increased by exogenous cysteine, but not cystine. In T-cells, the membrane transport activity for cystine is ten-fold lower than that for cysteine. As a consequence, T-cells have a low baseline supply of cysteine, even under healthy physiological conditions. The cysteine supply function of the macrophages is an important part of the mechanism which enables T-cells to shift from a GSH-poor to a GSH-rich state.
The importance of the intracellular GSH concentration for the activation of T-cells is well established. It has been reported that GSH levels in T-cells rise after treatment with GSH; it is unclear whether this increase is due to uptake of the intact GSH or via extracellular breakdown, transport of breakdown products, and subsequent intracellular GSH synthesis. Decreasing GSH by 10-40% can completely inhibit T-cell activation in vitro. Depletion of intracellular GSH has been shown to inhibit the mitogenically-induced nuclear size transformation in the early phase of the response. Cysteine and GSH depletion also affects the function of activated T-cells, such as cycling T-cell clones and activated cytotoxic T-lymphocyte precursor cells in the late phase of the allogeneic mixed lymphocyte culture. DNA synthesis and protein synthesis in IL-2 dependent T-cell clones, as well as the continued growth of preactivated CTL precursor cells and/or their functional differentiation into cytotoxic effector cells are strongly sensitive to GSH depletion.
Glutathione status is a major determinant of protection against oxidative injury. GSH acts on the one hand by reducing hydrogen peroxide and organic hydroperoxides in reactions catalyzed by glutathione peroxidases, and on the other hand by conjugating with electrophilic xenobiotic intermediates capable of inducing oxidant stress. The epithelial cells of the renal tubule have a high concentration of GSH, no doubt because the kidneys function in toxin and waste elimination, and the epithelium of the renal tubule is exposed to a variety of toxic compounds. GSH, transported into cells from the extracellular medium, substantially protects isolated cells from intestine and lung against t-butylhydroperoxide, menadione or paraquat-induced toxicity. Isolated kidney cells also transport GSH, which can supplement endogenous synthesis of GSH to protect against oxidant injury. Hepatic GSH content has also been reported to increase (i.e. to double) in the presence of exogenous GSH. This may be due either to direct transport, as has been reported for intestinal and alveolar cells, or via extracellular degradation, transport, and intracellular resynthesis.
The nucleophilic sulfur atom of the cysteine moiety of GSH serves as a mechanism to protect cells from harmful effects induced by toxic electrophiles. It is well established that glutathione S-conjugate biosynthesis is an important mechanism of drug and chemical detoxification. GSH conjugation of a substrate generally requires both GSH and glutathione-S-transferase activity. The existence of multiple glutathione-S-transferases with specific, but also overlapping, substrate specificities enables the enzyme system to handle a wide range of compounds. Several classes of compounds are believed to be converted by glutathione conjugate formation to toxic metabolites. For example, halogenated alkenes, hydroquinones, and quinones have been shown to form toxic metabolites via the formation of S-conjugates with GSH. The kidney is the main target organ for compounds metabolized by this pathway. Selective toxicity to the kidney is the result of the kidney's ability to accumulate intermediates formed by the processing of S-conjugates in the proximal tubular cells, and to bioactivate these intermediates to toxic metabolites.
The administration of morphine and related compounds to rats and mice results in a loss of up to approximately 50% of hepatic GSH. Morphine is known to be biotransformed into morphinone, a highly hepatotoxic compound, which is 9 times more toxic than morphine in mouse by subcutaneous injection, by morphine 6-dehydrogenase activity. Morphinone possesses an α,β-unsaturated ketone, which allows it to form a glutathione S-conjugate. The formation of this conjugate correlates with loss of cellular GSH. This pathway represents the main detoxification process for morphine. Pretreatment with GSH protects against morphine-induced lethality in the mouse.
The deleterious effects of methylmercury on mouse neuroblastoma cells are largely prevented by co-administration of GSH. GSH may complex with methylmercury, prevent its transport into the cell, and increase cellular antioxidant capabilities to prevent cell damage. Methylmercury is believed to exert its deleterious effects on cellular microtubules via oxidation of tubulin sulfhydryls, and by alterations due to peroxidative injury. GSH also protects against poisoning by other heavy metals such as nickel and cadmium.
Because of its known role in renal detoxification and its low toxicity, GSH has been explored as an adjunct therapy for patients undergoing cancer chemotherapy with nephrotoxic agents such as cisplatin, in order to reduce systemic toxicity. In one study, GSH was administered intravenously to patients with advanced neoplastic disease, in two divided doses of 2,500 mg, shortly before and after doses of cyclophosphamide. GSH was well tolerated and did not produce unexpected toxicity. The lack of bladder damage, including microscopic hematuria, supports the protective role of this compound. Other studies have shown that co-administration of GSH intravenously with cisplatin and/or cyclophosphamide combination therapy, reduces associated nephrotoxicity, while not unduly interfering with the desired cytotoxic effect of these drugs.
GSH has an extremely low toxicity, and oral LD50 measurements are difficult to perform due to the sheer mass of GSH, which has to be ingested by the animal in order to see any toxic effects. GSH can be toxic, especially in cases of ascorbate deficiency, and these effects may be demonstrated in, for example, ascorbate deficient guinea pigs given 3.75 mmol/kg daily (1,152 mg/kg daily) in three divided doses, whereas in non-ascorbate deficient animals, toxicity was not seen at this dose, but were seen at double this dose.
There is a need in the art for other compounds and therapeutic aspects to treat a number of diseases that are linked to oxidative stress and the presence of free oxygen radicals and associated disease pathogenesis in cells and tissues. Needed are antioxidant compounds, other than GSH, that are safe and even more potent, to overcome high oxidative stress in the pathogenesis of diseases. Ideally, such compounds should readily cross the blood-brain barrier and easily permeate the cell membrane. Antioxidants such as vitamins E and C are not completely effective at decreasing oxidative stress, particularly because, in the case of vitamin E, they do not effectively cross through the cell membrane to reach the cytoplasm so as to provide antioxidant effects.